The accuracy of a continuous time filter depends, in part, upon the tolerances of its components. For example, there is a practical limit to the precision to which integrated components (e.g., resistors, capacitors and transistors) can be fabricated. Fabrication process variances may prevent two components within a single filter from having the same specified value.
Consequently, two filters, sharing a common specification, may not perform identically and may have different center or tuned frequencies. Additionally, component values (and, therefore, the tuned frequency) may change during operation of a filter due to temperature fluctuations and aging. Finally, there may be circumstances in which it is desirable to intentionally alter the tuned frequency.
Briefly, tuning a filter can be accomplished by adjusting the characteristics of the filter's capacitors or transistors by means of a tuning bias. If automatic tuning is desired, such as to offset the effects of temperature variations and aging, feedback circuitry is employed. A phase locked loop (PII.) is commonly used in such circuitry with either a voltage controlled oscillator (VCO) or a voltage controlled filter (VCF). These circuits detect the phase difference between a reference clock signal with a predetermined frequency and a reference signal filtered by a reference filter having the same nominal specifications as the operational filter. The phase difference is minimized by altering the tuning bias of the reference filter. Similar adjustments are also made to the tuning bias of the operational filter under the assumption that variations (such as those which are due to temperature or aging) in the operational filter will also be present in the reference filter, and to the same extent.
For the basic assumption to be valid, the two filters should be precisely matched and in close physical proximity to each other. However, due to the varied fabrication process and other variances which an automatic tuning technique is designed to overcome, such precision is expensive and difficult to achieve, especially at high frequencies. Additionally, the reference clock frequency should be outside the pass band of the filter to reduce adverse coupling of the reference clock signal (and any noise associated with it) with the signal being filtered.
A switched capacitor tuning technique has also been proposed for altering the characteristics of a filter which relies upon the frequency dependency of a switched capacitor resistor. A clock signal controls a set of switches (such as MOS transistor switches) which permit the capacitor to charge and discharge at the clock frequency rate. Changing the clock frequency changes the effective resistance of the capacitor, thereby tuning the filter. Such a circuit requires a high frequency clock or a very large capacitor, which may not be able to be implemented in integrated form.
It should be particularly noted that circuits incorporating these techniques, which employ a reference signal and in which the tuning occurs while the filter is in operation, should include means to prevent noise from the reference signal from passing into the biasing signal and into the information signal itself.
Because complete filtering of such noise may not be possible, the performance of the filter can be adversely affected.